A common type of small cryogenic refrigerator in use today is one which makes use of the Gifford-McMahon (G-M) operating cycle. This cycle is used in both single and multiple-stage configurations. A basic description of the G-M operation is set forth in U.S. Pat. No. 3,045,436, issued on July 24, 1962 to W.E. Gifford and H.O. McMahon. Other apparatus configurations using G-M principles of operation are also described, for example, in U.S. Pat. Nos. 3,119,237 and 3,421,331, issued on Jan. 28, 1964 and Jan. 14, 1969 to W.E. Gifford and to J.E. Webb, respectively.
In such systems, no heat energy is transferred from the expanding fluid through the performance of mechanical work external to the refrigerator. Thus, while a moveable displacer element is periodically moved within the appartus to provide for an expansion chamber, this element is not arranged so as to produce an external mechanical energy exchange. Rather, as would be well known to those in the art, the displacer moves mass and mechanical energy between confined fluid volumes.
In such an approach, the confined fluid volumes on either end of the displacer are connected by a heat exchange passage, often called a thermal regenerator. The thermal regenerator undergoes the same pressure cycling as the confined fluid volumes. In such a configuration, the heat energy is normally fully stored for a half cycle in the regenerator matrix, which requires the regenerator matrix to have a relatively large heat capacity. In totally regenerative cycles, such as in the G-M approach, the pressure ratio is effectively limited by the gas volume in the regenerator, which volume must be large enough so that the low-pressure-flow pressure drop through the regenerator matrix is not excessive.
Another type of refrigerator well-known to the art and similar in appearance to the Gifford-McMahon type, but different in operation, is one which uses a Solvay cycle of operation. Both the G-M and Solvay techniques use valved, regenerative operating cycles, but the Solvay cycle performs mechanical work extraction from the refrigerant fluid. Thus, the expanding gas at the cold end of a piston performs work on a drive mechanism attached to the other end of the piston. Because of this operation, a Solvay refrigerator requires a high pressure gradient over the piston seal, while the G-M approach, with no work interaction, incorporates only a low pressure gradient over the displacer seal. While the high pressure gradient seal is a significant reliability drawback, the Solvay cycle is normally more efficient than the G-M cycle.
Common regenerator materials have a heat capacity that diminishes at very low temperatures. For this reason, the Gifford-McMahon or Solvay cycles are not capable of producing effective cooling at, for example, liquid helium temperatures, even when multiple stages are used. To reach liquid helium temperatures, a second thermodynamic operating cycle, such as a well-known Joule-Thomson operating cycle, must be used in combination with a Gifford-McMahon cycle, for example. The Joule-Thomson cycle of operation utilizes a pre-cooling counterflow heat exchanger and an expansion valve (commonly referred to as a Joule-Thomson valve). Since neither the G-M, the Solvay, nor the Joule-Thomson cycle is capable of reaching liquid helium temperatures independently, in order to reach liquid helium temperatures, it has been suggested that various appropriate combinations of such techniques be used. Thus, a number of G-M stages can be used to provide for a pre-cooling of the helium gas before it is supplied to the counterflow heat exchanger of the Joule-Thomson operating cycle in preparation for the expansion of the gas during the Joule-Thomson operation. Such a combined cycle configuration could be capable of producing cooling down to liquid helium temperatures. While such a system has been commercially available, it has some severe drawbacks. For example, mechanically combining the two configurations results in a relatively complex physical configuration which is difficult to manufacture, resulting in a system which is often prohibitively expensive for many, if not most, applications. Further, such systems have poor reliability due to clogging of the Joule-Thomson valve and to the difficulty in controlling the operation of such valve. Moreover, the optimal mean cycle pressures and pressure ratios for the two cycles are not compatible, so that the combination requires a specially designed compressor configuration, thereby further increasing the cost and difficulty of manufacture.
A further refrigeration method has been described in U.S. Pat. No. 4,862,694 issued on Sept. 3, 1989 to J.A. Crunkleton and J.L. Smith, Jr. The patent discloses a method for attaining refrigeration at liquid helium temperatures in a relatively simple and compact configuration. One embodiment of the technique discussed therein incorporates a counterflow heat exchange operation which in a preferred embodiment thereof is integral with the piston-cylinder structure thereof. Mechanical work is extracted from the refrigerant gas during the expansion process. One exemplary cycle of operation for a single-stage configuration can be described as follows.
When the piston is in its minimum volume position, an intake valve at room temperature opens to allow high-pressure gas at room temperature to enter the gap between the piston and cylinder. While the gap is charged to full pressure, the intake valve remains open and the piston begins to move, thereby drawing more high pressure gas into the expansion space created below the piston. The constant high-pressure intake continues until the inlet valve is closed. At this time, the expansion portion of the cycle begins. When the piston is at the maximum expanded volume position, a cold exhaust valve opens and the blow-down portion of the exhaust occurs. Movement of the piston then decreases the expansion volume in order to exhaust gas at constant pressure. At the appropriate piston position, the exhaust valve closes and recompression begins. When the piston reaches a position near minimum volume, the intake valve opens and the cycle is repeated.
The gas, which has been exhausted through the cold exhaust valve, enters a surge volume. This volume, coupled with the flow restriction in the low-pressure return flow path between the cylinder and outer shell, results in an effective resistive-capacitive circuit flow arrangement. Accordingly, the mass flow rate in the return flow path is more nearly constant during the cycle period. The gas exits the surge volume and enters the low-pressure return flow passage between the cylinder and outer shell. As the low pressure gas is travelling at a nearly constant rate between the cylinder and the outer shell, it is exchanging heat with gas flowing between the piston and cylinder. Highly efficient counterflow heat transfer occurs to cool the high pressure gas entering the expansion space in preparation for the next expansion stroke.
Such a method of refrigeration is also described as one which can be performed in multiple stages. Typically, high pressure gas enters at room temperature and is pre-cooled as it flows through one or more upper expansion volume stages on its way to the coldest expansion volume stage. The piston is arranged to have a stepped configuration so that, as it moves during the intake and expansion portions of the cycle, such movement would create a number of expansion volumes of varying temperature. During the exhaust phase, gas would flow through the exhaust valves at each of the stages of expansion.
While the system described in the aforesaid Crunkleton and Smith patent operates satisfactorily, it requires a number of "cold" valves, i.e., valves which operate at low temperatures, one at each operating stage. Such valves not only are costly, but also have lower reliability than valves designed for use at warmer temperatures, e.g., at or near room temperature. It is desirable to provide an improved technique which produces effective and reliable operation at extremely low temperatures and which has relatively low manufacturing and operating costs.
The present invention recognizes that, while counterflow heat exchange is essential for attaining liquid helium temperatures at the coldest expansion stage, it is not required for the warmer stages. At temperatures above about 20.degree. K., for example, the heat capacity of the heat exchanger materials is large compared to the net enthalpy flux of the helium through the heat exchanger over a half cycle so that the regenerative heat exchange operation can be efficient above about 20.degree. K. but is much less efficient below such temperature.
The refrigeration method of this invention combines the simplicity and efficiency of regenerative heat exchange for the warmer stages of a multi-stage cooling device with highly efficient counterflow heat exchange at the colder stage or stages. In addition, the warmer expansion stages no longer require individual cold exhaust valves at each expansion stage, thereby increasing reliability of the system and lowering its cost.